The present photocathode is a device for the generation of a beam of electrons. One prior art method for the generation of an electron beam is a thermionic cathode, such as a Pierce electron gun or a Brillouin electron gun, both of which utilize a cathode heated to a sufficiently high temperature to release electrons through thermionic emission. Unlike a traditional thermionic cathode, a photocathode generates an electron beam when a high intensity optical source such as a laser impinges onto a cathode, relying on the quantum efficiency (QE) of the photocathode target material to convert the incoming photons into an electron beam. One advantage of the photocathode is the ability to operate at any temperature, and the ability to generate electrons for picosecond time intervals by modulating the laser with picosecond pulses.
FIG. 1 shows a prior art cesium photocathode 100, having a photoelectric surface 116 which is impinged by photons from an optical source shown as laser beams 106 and 108, and the photoelectric effect causes the release of electrons at various release angles 102, 104, 118, as shown. While a cesium photocathode has improved quantum efficiency, the surface is sensitive to contamination, and known prior art cesium coatings have a high evaporation rate, which results in an undesirably short cathode lifetime, as the loss of surface cesium results in the associated loss of quantum efficiency. Another problem is that the quantum efficiency of a cesium cathode is dependant on the cesium coating thickness.
It is desired to provide a long lifetime cesium photocathode with a high quantum efficiency. It is also desired to provide a method to optimize the quantum efficiency of a cesium coated photocathode, and maintain the operation of the photocathode at an optimum quantum efficiency over time.